Alliance for Green
Heat
April 1, 2014
Conventional wisdom says that cat stoves burn cleanest on a
low air setting and non-cats burn cleanest on a high air setting. Conventional wisdom also says that pellet stoves
are cleaner than wood stoves. However, the
database used by industry and the EPA to analyze stoves shows that the reality
is much more complicated than conventional wisdom suggests. And, the implications for designing and
testing cleaner stoves that hit a 1.3 standard, or whatever number the EPA
arrives at, may be significant.
This analysis shows that stoves in all categories-cat,
non-cat and pellet – usually burn dirtiest on their high air setting, which
under a weighted average in the existing NSPS, is largely discounted. Under the proposed NSPS that high burn rate
becomes all-important, and will be the focus of most cat, non-cat and pellet stove
testing. This may lead to manufacturers
trying to reduce the high air settings to get their stoves to pass. This in turn could increase start-up
emissions and make it harder to get stoves up to temperature quickly.
This also raises the question about whether it makes sense to test stoves at their highest air setting, when homeowners usually use them at the lowest air setting. With 40% of non-cat stoves emitting the highest emissions in Category 4, should the new NSPS really test them based on Category 4 emissions and completely ignore Category 1 emissions? We think that weighting of emissions can still make sense under the new NSPS, although the weighting of wood stoves may be different from the weighting of pellet stove emissions, based on data of where homeowners typically use their respective type of stoves. Weighting should at least be considered to include all the test burns required by the NSPS. If four test burns are required, one high and one low and two more at the dirtiest burn rate, they could all be weighted equally.
This analysis is based on a database of 147 stoves, compiled
by the Hearth Patio and Barbecue Association (HPBA) in 2010. It became public in January of this year when
the EPA released it among the scores of documents they used to determine what
emission levels to set for wood and pellet stoves in the new NSPS.
Summary notes:
· * When the 5G correction factor is removed, as it
will be under the new NSPS, more stoves will likely be able to pass stricter
limits than what is often being reported.
Many who cite figures about how many stoves can pass the new NSPS limits
appear to be unaware of this.
· * Five stoves – 2 catalytic, 2 non-catalytic and 1
pellet–appear to be able to pass a 1.3 gram per hour (g/hr) standard with the
5G correction using crib wood. More will likely pass without it.
· * If the EPA settled on a 2 g/hr standard, 16
stoves would appear to pass: 8 pellet, 4 catalytic and 4 non-catalytic.
· * On average, non-cat stoves tend to burn cleanest
on Category 3, the medium high burn rate and dirtiest on Category 4, the high
burn rate.
· * Cat stoves burn consistently cleanest on
Category 1, the low burn rate and dirtiest on Category 4.
· * Pellet stoves burn almost equally cleanly on
Categories 1, 2 and 3, but are significantly dirtier on Category 4.
· * To pass future standards, many manufacturers may
look to reducing the air in Category 4, which could have an impact on efficiency
and may have effect of reducing maximum BTU output.
· * By testing at its dirtiest burn rate, many wood
stove would be tested at burn rates which the consumer does not often use. This may make testing less characteristic of
real world use, rather than more.
· * There is a very slight negative correlation
between firebox size and emissions with non-cat stoves showing slightly lower
average emissions from larger fireboxes.
Larger non-cat stoves are known to be harder to tune and get to pass.
· * There is a very slight positive correlation
between firebox size and emissions with catalytic stoves showing slightly
higher average emissions from larger fireboxes.
· * On the EPA list of certified stoves, there is a
high concentration of stoves that tested just less than 4.5 g/hr, and
relatively few that tested above 4.5.
This may indicate that stove companies are able to fine-tune their stoves
to hit stricter emission targets.
The Database
The main database used by industry and the EPA has extensive
details about 147 stoves, showing emission rates at the 4 burn levels. It was compiled by Bob Ferguson, a consultant
for HPBA who collected data from manufacturers who agreed to share it. The EPA
independently also has this data and more.
Legally, emissions data is not protected by the confidential business
information (CBI) label, but all manufacturers submit it as CBI, which then
requires the EPA to undergo a lengthy process to remove the label. The data was
compiled in 2010, so it’s a bit out of date and not an exhaustive list, but it still
provides useful and important data to understand how a 1.3 g/hr standard may
impact the stove industry in 5 or 8 years, when and if that standard becomes
law. There are up to 100 stoves not on
this list, including many that became certified after 2010, which are being
analyzed by the EPA.
5G, 5H and the
Correction Factor
One of the biggest hidden features of the NSPS is that
stoves at 3.7 or 4.4 g/hr under the existing NSPS could come out a gram or even
2 grams less under the new NSPS. This is
because the EPA is getting rid of an adjustment or correction factor that has
been used for stoves tested under certain test methods. Most of the industry does not realize this
and it makes the numbers proposed by the EPA appear stricter than they actually
are. In this analysis here, we are just
using the numbers in the existing NSPS and we have not re-adjusted them. If we
had, it would make many of them show much lower emission numbers. (For more
info on this, see the emission testing method discussion here.) Seventy-two of the 147 stoves on this
list used some version of 5G (5G1, 5G2 or 5G3). Twenty-nine did not specify
whether they used 5G or 5H.
Stoves that can meet
1.3 grams per hour
This analysis is solely about meeting 1.3, or whatever
standard the EPA arrives at, using crib wood testing. Crib wood testing will continue to be used
for five years before transitioning to cord wood. Once manufacturers start designing for
cordwood testing, stoves may operate in people’s homes more like they were
tested in the lab, and become cleaner.
For the next five years, the EPA will maintain a 4.5 standard that average stoves can meet. The proposed 1.3 standard to take effect in five years purposefully seeks to achieve the best demonstrated technology and get manufacturers to change the design of their stoves.
For the next five years, the EPA will maintain a 4.5 standard that average stoves can meet. The proposed 1.3 standard to take effect in five years purposefully seeks to achieve the best demonstrated technology and get manufacturers to change the design of their stoves.
Of the 147 stoves on the list, 5 of them tested under 1.3
g/hr on low and high rates and are likely to pass the proposed new EPA
standards using crib wood – but not necessarily cordwood. The Alliance commented on how the EPA can set a standard for cordwood. Two of them are cat stoves, 2 are non-cat and one is a pellet stove.
Overall Average
Emission Rankings
The overall average weighted emission rate of all the stoves
on the list is 2.01 for pellet stoves, 2.05 for cat stoves and 3.51 for non-cat
stoves. On average, all three categories
of stoves were dirtiest on Category 4, which is the highest air setting and the
most BTU output. Cat stoves were the
only type that was uniformly cleaner at Category 1 and uniformly dirtier at
Category 4. For non-cats, the cleanest
average burn was Category 3, but there was only about a one g/hr difference
between all 4-burn rates. Conversely,
cat stoves were consistently and significantly cleaner at one burn rate
compared to others, with more than a 2.5 g/hr range. Pellet stoves were slightly cleaner on
Category 2, but Categories 1, 2 and 3 were very similar. There was about a 1.5-g/hr range between
cleanest and dirtiest burns.
It is important to know that EPA emissions numbers do not
equally average the 4 burn rates to come up with a final number. They are a “weighted average,” so that the high burn rate counts
for very little and the low burn rate counts a lot. Since the Category 4 high burn doesn’t
affect the weighted average much, most manufacturers don’t pay that much attention
to it. This is one reason why the
average Category 4 burns were the dirtiest for all stove types. On the other
hand, the low burns are heavily weighted, so stoves are designed to perform
well at those levels. High burns that previously
counted for 1-10% of a weighted average will now count heavily. The good
news for stove manufacturers is that high burn emissions are often easier and
less costly to reduce than low burn emissions.
But the impact in the field is likely to be increased emissions during
start-up.
Its
also relevant that the median numbers for each burn rate for each stove type
are lower than the average, showing that there are more stoves on the cleaner
end of the spectrum.
Cat Stove Emission
Characteristics
Cat stoves were the only stove type to have an average under
1.3 on any burn category. Cat stoves had
an average of 1.1 g/hr on Category 1, the low burn. But under the new EPA proposal, stoves have
to meet 1.3 on their dirtiest setting, which will be Category 4 for cat
stoves. In this stove sample, the
average on Category 4 was 3.6.
Under the proposed new method of testing at the dirtiest
burn level, stoves that consistently have the greatest range between Categories
1 and 4 may be penalized, and stoves that have the least range may
benefit. While cat stoves have the
cleanest burn at their “sweet spot” which helped them pass with flying colors over
the past 2 decades, they will lose some of that advantage under the new rule
which does not average in the clean burns, much less give them greater weight.
Of the 15 cat stoves:
· * 13 burned cleanest on Category 1, the low burn
rate
· * 12 burned dirtiest on Category 4, the high burn
rate
· * The highest emitter was 9.7 g/hr on Category 4
Non-cat Emission
Characteristics
Non-cats had the dirtiest average weighted burn rate of 3.5
and had the dirtiest average on any single burn category. They had an average of 4.04 on high burn, and
the cleanest average was 2.9 on medium high.
This contradicts the much-repeated conventional wisdom that non-cats are
cleanest on their highest air setting and dirtiest on the lowest air
setting. Under the new NSPS, these
stoves will usually have to be tested at the highest air setting, which is
their dirtiest and have the furthest to come down towards 1.3. This may result
in non-cats (and cats) being tested at burn rates that homeowners don’t often
use. Unlike cat stoves, that are
consistently cleanest on Category 1 and consistently dirtiest on Category 4,
the following table shows that cat stoves do not show any similar consistency:
We question whether this data supports the EPA’s proposal to
test on a stove’s dirtiest burn rate.
Another option would be to maintain the weighted average for wood stoves
(not pellet stoves) but put a cap on emissions on any test run as the EPA
proposes to do with outdoor boilers.
Thus, even if the EPA finalized on 1.3 or 2.0 g/hr, the stove could not
emit more than 3 or 4 g/hr on any burn rate.
Out of the 110 non-cats on the list:
·
8 were cleanest on Category 1
·
28 were cleanest on Category 4
·
54 were dirtiest on Category 4, the highest burn
rate, and
·
33 were dirtiest on Category 1, the low burn
rate
·
The highest emitter was 17.4 g/hr on Category 4
Pellet stove Emission
Characteristics
As noted above, the cleanest weighted average included one pellet
stoves that emitted under 1.3 g/hr on high and low burn rates. Even though
nearly a third of pellet stoves had a weighted average under 1.3, with the
weighting removed and testing focused on the dirtiest emission rate, many
pellet stoves would have to redesign to get their high burn rate emissions
down. This is why many manufacturers are currently getting the pellet stove
certified so they have a 5-year certificate and won’t have to retest under the
new testing protocol for 5 more years.
Emissions were virtually flat on Categories 1, 2 and 3 (between
1.65 and 1.8) and about the same number of stoves had their cleanest run on
Category 1, 2 and 3. Pellet stoves were
consistently dirtiest in Category 4, where emissions jumped to an average of 2.9.
One pellet stove put out 11.9 g/hr on Category 4. However, many experts believe that pellet
stoves have a lot of room for improvement and have been intentionally
“de-tuned” to meet the 35 to 1 air to fuel ratio.
Of the 22 pellet stoves:
·
10 burned cleanest on Category 1, the low burn
rate
·
12 burned dirtiest on Category 4, the high burn
rate
·
5 burned dirtiest on Category 1
·
The dirtiest emitter was 11.9 on Category 4
There is greater certainty around pellet stoves, as they are
not undergoing such a drastic switch in testing methods from crib wood to
cordwood. Their test fuel is not
remaining the same however, as the EPA will start to require the use of PFI
certified pellets, which may perform slightly different than the pellets used
before.
If the EPA were ultimately to set a 2 g/hr standard after 5
years, this database shows that pellet stoves would be the category that shines
between 1.3 and 2 g/hr, with about a third of them emitting less than 2 g/hr on
all four burn rates. If the EPA were to
set a 2.0 limit, there are hardly any more cat or non-cat stoves that would be
less than 2 g/hr four burn runs on this list (more would likely pass after
taking away the 5H correction).
Implications for
Efficiency
Under the proposed NSPS, there will be no minimum efficiency standard, but manufacturers will have to test and report efficiencies using the CSA 415.1 stack loss efficiency test. This is a method that measures how hot the flue gasses are coming out of the stack, compared to the heat that was transferred from the stove to the room. All other things being equal, the hotter the gas coming out the chimney, the less efficient the stove.
Under the proposed NSPS, there will be no minimum efficiency standard, but manufacturers will have to test and report efficiencies using the CSA 415.1 stack loss efficiency test. This is a method that measures how hot the flue gasses are coming out of the stack, compared to the heat that was transferred from the stove to the room. All other things being equal, the hotter the gas coming out the chimney, the less efficient the stove.
Stoves tend to burn more efficiently at lower burn rates and less efficiently at higher burn rates. To design a stove that meets 1.3, or whatever number the EPA arrives at, on high burn, many manufacturers may focus R&D on lowering their highest burn rate. Under this scenario, a stove’s efficiency may rise.
However, if
efficiency calculations under B415.1 are done using a weighted average of all
four burn rates, that data will not even be available and efficiency will only
be calculated using high and low burn rate data.
One
negative implication of reducing air in Category 4 is that stove start up may
be more difficult and could result in greater emissions as Category 4 is most
consistently used during the start up period. This problem could be reduced if
the operator leaves the door slightly ajar, which is already a very common
practice and recommended as an option in some owner’s manuals. However, this cannot be done during
certification testing.
Pellet
stoves are likely to see the most rapid rise in efficiencies because many of
them are currently exempt because of the 35 to 1 air to fuel ratio
exemption. By using this exemption from
certification, manufacturers have penalized the efficiency of many pellet
stoves. By removing that exemption to certification
in the proposed NSPS, those stoves will have to reduce airflow through the
combustion chamber, which can significantly raise efficiency. A 5 – 20% rise in efficiency, or more, is
possible for many exempt stoves. This
will result in significant fuel savings for thousands of consumers.
Tuning a
pellet stove for maximum efficiency could cause problems in stoves that are
vented through the wall instead of through the ceiling. Pellet stoves are tested with a vertical
stack set-up and a side vented unit will not have the benefit of that increased
draft. The lower efficiency and higher airflow
of some existing pellet stoves can help them in the field when they are side vented.
Cat
Stove Emission Correlation
We found that there is a small positive
correlation between the usable firebox volume and EPA weighted emissions. This
means that as the volume of the firebox increases, so does the emission of the
stove in g/hr. The value of R (correlation) is 0.3356. Although technically a
positive correlation, the relationship between the two variables is weak (the
nearer the value is to zero, the weaker the relationship) using an alpha of
0.05, the correlation is not deemed to be statistically significant. The
scatter plot below depicts the slight positive slope.
Non-cat
Stove Emission Correlation
Distribution
of Tested Emissions in Non-Cat Stoves
As proven by the above, wood itself and its burning is just too complicated for anyone to do properly. So the best is just to butt out in crowded areas. Wear layers of clothes in a colder house.
ReplyDeleteHey Vic,
ReplyDeleteI would like to respectfully disagree. Burning wood is definitely a big challenge, but it can be done in a good and clean way. Although it certainly isn't most of them, a lot of stoves out there can meet these new standards, and new stoves in the works will be able to do it as well.
It's going to require more work by manufacturers, there is no denying it. But the result is an inexpensive, local, renewable, and clean way to heat our homes. I think that is worth the effort.
John, I applaud you for this rather complicated article regarding efficient burns and the implications for the proposed NSPS. I would hope EPA would test for heating efficiency by using the CSA 415.1 stack loss method and have the results posted for each stove. It is important that customers have this knowledge. I am using this method to test a stove design I am working on. It's simple and revealing. I discovered what your article revealed.....That a burn with reduced draft increased the heating efficiency of my stove. Richard J
ReplyDelete